1. Field of the Invention
The present invention relates to an improved film forming apparatus for continuously forming a functional film, particularly, a crystalline or non-monocrystalline semiconductor which is appropriately usable as an electrophotographic photosensitive member, a photovoltaic device, an image input line sensor, an image pickup device, and a semiconductor device such as TFT, on a substrate with the microwave plasma CVD method.
2. Related Background Art
Heretofore, non-monocrystal deposit films such as amorphous silicon, for example, amorphous silicon compensated with hydrogen and/or halogen (e.g., fluorine or chlorine) [hereinafter abbreviated as A-Si(H,X)], or crystalline deposit films such as diamond thin film, have been proposed as a device member for use in a semiconductor device, an electrophotographic photosensitive member, an image input line sensor, an image pickup device, a photovoltaic device, and other various electronic devices or optical devices, and some of them are practically used. And it is known that those deposit films are formed with the plasma CVD method, that is, the method of decomposing a source gas with the glow discharge caused by the direct current or radio frequency, or the microwave to form a deposit film on a substrate such as glass, quartz, heat-resistive synthetic resin film, stainless, or aluminum, in which various apparatuses for use with that method have been proposed.
In particular, recently, the plasma CVD method using the microwave glow discharge decomposition or the microwave plasma CVD method (hereinafter abbreviated as .mu.W-PCVD method) has also been noted in industries.
The .mu.W-PCVD method has an advantage of offering a higher deposition speed and a higher source gas utilization efficiency than other methods. One example of .mu.W-PCVD apparatus using such advantage is disclosed in Japanese Patent Application No. 60-186849. The apparatus as described in that patent increases the gas utilization efficiency by disposing a substrate so as to surround introducing means of the microwave energy to an inner chamber (or discharge space).
Also, in Japanese Patent Application No. 61-283116, an improved microwave technique for fabricating a semiconductor member has been disclosed. That is, the above patent discloses a technique of improving the characteristics of deposit film in such a manner as to provide an electrode for the control of plasma potential in a plasma space, and deposit the film while controlling ion bombardment onto the deposit film by applying a desired voltage to the electrode.
With these conventional methods, it was enabled to fabricate a relatively thick photoconductive material at certain high deposit speed and in utilization of the efficiency of source gas.
The deposit film forming apparatus with such an improved .mu.W-PCVD method has typically an apparatus constitution as shown in a typical longitudinal cross-sectional view of FIG. 3A and a typical transversal cross-sectional view of FIG. 3B (FIG. 3B is a typical transversal cross-sectional view of the apparatus as shown in FIG. 3A 9).
In FIGS. 3A and 3B, 301 is a reaction vessel having a vacuum airtight structure. 302 is a microwave introducing dielectric window formed of a material capable of efficiently transmitting the microwave electric power into the reaction vessel and preserving the vacuum airtight (e.g., quartz glass or alumina ceramics). 303 is a transmission section of the microwave electric power, consisting of a waveguide and connected via a stub tuner (not shown) and an isolator (not shown) to a microwave power source (not shown). The dielectric window 302 is sealed airtightly to a wall of the waveguide 303. 304 is an exhaust pipe having one end opening to the reaction vessel 301 and the other end communicating to an exhaust apparatus (not shown). 306 is a discharge space surrounded and formed by a plurality of cylindrical substrates 305. 308 is an electrode for supplying an external electric bias to control the plasma potential, to which the D.C. or A.C. voltage is applied from a power source 309. Note that each cylindrical substrate is installed on a cylindrical holder containing a heater 307, each holder being rotated properly by drive means (rotary motor) 310.
The formation of deposit film with such a conventional deposit film forming apparatus can be performed in the following manner.
First, the reaction vessel 301 is exhausted via the exhaust pipe 304 by means of a vacuum pump (not shown) to adjust the pressure or internal pressure within the reaction vessel to less than about 1.times.10.sup.-7 Torr. Then, the substrate 305 is heated and retained at a temperature suitable for the film deposition, with the heater 307. Thus, a source gas, for example, silane gas or hydrogen gas if an amorphous silicon deposit film is formed, is introduced via a gas inlet tube (not shown) into the reaction vessel 301. Then the microwave having a frequency of 500 MHz or more, preferably 2.45 GHz, is produced with the microwave power source (not shown) and introduced via the microwave waveguide 303 and the dielectric window 302 into the reaction vessel 301. At the same time or in parallel, the D.C. voltage from the power source 309, for example, is applied to the electrode 308 provided in the discharge space 306 as the external electric bias. Thus, the source gas is excited and dissociated by the microwave energy in the discharge space 306 surrounded and formed by a plurality of cylindrical substrates 305, so that the deposit film can be formed on all the surfaces of cylindrical substrates 305. At this time, by rotating each of the cylindrical substrates 305 around a central axis in the generatrix direction, the deposit film is formed on an entire surface of individual cylindrical substrate.
With such a conventional deposit film forming apparatus based on the .mu.W-PCVD method, it is possible to obtain a deposit film having practical characteristics and uniformity at a certain film deposition speed. However, with such conventional deposit film forming apparatus based on the .mu.W-PCVD method, there is a problem that significant skill may be required to obtain the deposit film in uniform quality and meeting the requirements of optical and electrical characteristics steadily, stably and at high yield, particularly in the field of high film deposition speed, as for example, in the fabrication of the deposit film having a large area and a relatively large film thickness such as an electrophotographic photosensitive member.
That is, in order to form a desired deposit film on a substrate of large area at high film deposition speed, with the utilization efficiency of source gas being maintained at high value, as is the case with the electrophotographic photosensitive member, the following conditions must be satisfied.
(1) It is necessary to deposit the film on a substrate of large area (in particular 3000 mm.sup.2 or more) at high speed, and maintain excellent characteristics whereby a larger amount of source gas flow, a larger microwave energy and a lower pressure as compared with the conventional example are requisite. PA1 (2) It is necessary to avoid the occurrence of defects over a large area (in particular 3000 mm.sup.2 or more) and the peeling of the portion on which the deposit film is formed. PA1 (3) It is necessary to make uniform the electrophotographic characteristics uniform over a large area (in particular 3000 mm.sup.2 or more) up to the level at which there is no visible density difference on the image. To this end, the high uniformity of film thickness and quality that is required in the entire image forming is on the substrate.
Generally, when the deposit film is formed using the plasma by introducing the microwave into the reaction vessel, the deposit film is divided into an area (power limit area) where the film formation seed is increased with increasing microwave power because the source gas has not been completely decomposed and an area (flow limit area) where the film formation speed is not changed with increasing microwave power because the source gas has been completely decomposed, if the microwave electric power is increased with the source gas fixed at a constant flow rate. In the power limit area, as the utilization of the efficiency of source gas is small while the source gas left undecomposed may have a bad influence in the reaction within a gas phase or on the surface of the substrate, the characteristics of the obtained deposit film are lowered. Also, in the flow limit area, the excess energy from the decomposition of source gas becomes the internal energy of decomposed species, in which with a larger microwave power for raising the surface mobility on the surface of substrate, superior characteristics of deposit film can be obtained. As the method of introducing the microwave, it is common to use a waveguide with a dielectric window superior in the transmission of the microwave. However, when such a large energy is introduced via the waveguide using the dielectric window, the energy density of microwave within the reaction vessel is rendered very large. Therefore, the dispersion of formed deposit film in the film thickness and film quality is liable to occur between the substrate portion located near the window and the other substrate portion. Furthermore, more defects of deposit film may arise because the peeling of relatively thick film may occur in the vicinity of the microwave introducing window, or the peeled film may stick to another area.
In order to suppress the reaction of deposit species in the gas phase, and maintain the uniformity of deposit film in the film quality and thickness in good condition, it is important to perform a uniform discharge at a pressure lower than 50 m Torr. However, in such a pressure range, the discharge may be less liable to occur if the pressure is lower, whereby it is difficult to maintain a stable discharge.
Particularly, in order to further improve the characteristics of deposit film, with a method of controlling the ion bombardment onto deposit species by providing an electrode to be commonly used with the microwave plasma discharge in the discharge space and applying a desired voltage to the electrode, there is a problem that abnormal discharge called a spark may be induced. This spark is more easily induced if the microwave power is larger. As the microwave introducing means is provided at an end portion of cylindrical substrate as previously described, the ion density near the microwave introducing means is increased, so that the non-uniformity of ion bombardment onto the surface of substrate may arise between the neighborhood of microwave introducing means and other areas, and the discharge becomes unstable owing to low pressure for maintaining the characteristics of deposit film, thereby causing the non-uniformity in the characteristics of deposit film.